1. Field of the Invention
This invention relates to a narrow-band laser apparatus.
2. Description of the Prior Art
The excimer laser has attracted attention as a light source for photolithography. The excimer laser can output a laser light beam having efficient power for exposing a photoresist film on a semiconductor wafer at several wavelengths between 353 nm to 193 nm through a laser medium comprising a noble gas, such as krypton and xenon gases, and halogen gas, such as fluorine and chlorine gases. Gain-band width of the excimer laser is approximately one nm which is too large for the exposure light source for photolithography. Band width output laser light of the excimer laser is about 0.5 nm (full width at half maximum). If such relatively broad band laser light is used as an exposing, an achromatic exposing optical system is necessary in exposing apparatus for photolithography. However, in ultraviolet region, less than 350 nm, achromatization is difficult because there are few types of optical materials which can be used for a focusing lens system. It is desired to narrow band width of the excimer laser used for the exposure light source whose band-width is around 0.005 nm. Such exposure light source enables a focusing lens system without achromatization to be used, so that simplification of the optical system of the exposing apparatus for photolithography and miniaturization and lowering cost of the exposing apparatus can be realized.
A laser apparatus for exposure which comprises an wavelength selection element provided in its optical resonator for narrowing bandwidth of laser light without attenuation of output power is described in Japanese patent application provisional publication No. 63-160287 which is described below.
FIG. 7 is a front view of the narrow-band excimer laser of the above-mentioned prior art. In FIG. 7, this prior art laser apparatus comprises an optical resonator including a total reflection mirror 102, a half mirror 103 and a discharge tube 101 provided in a light path of the optical resonator, and a Fabry-Perot etalon 104 as an wavelength selection element. In this laser apparatus, only light whose wavelength is selected by Fabry-Perot etalon 104 is amplified and oscillates, so that an extremely narrow-band laser light beam is obtained.
However in such excimer laser apparatus, there is a drawbacks that because there is high energy light continuously exists in the optical resonator, the wavelength selection element is deteriorate or deformed, so that the selection wavelength will change or output power will decrease. If such excimer laser apparatus is used as a light source for exposure, defected products of integration circuits are manufactured. In other words, maximum power of the excimer laser is limited by high-energy-light resistivity of the selection element.